Space exploration and other applications with diminished or zero gravity provides unique environments for deployment or dispensing of drops, droplets (small drops) or bubbles. These include experiments in fluid physics, drop physics and droplet combustion. They also include drops of material deployed for containerless processing studies. In these applications, the most common method of deploying a drop or a bubble is to use needles. Often these needles are arranged in matched pairs with the needles being juxtaposed in such a manner as to compensate for the lack of gravitational force. In the absence of gravity, the surface tension of the liquid becomes the dominant force which must be opposed in order to deploy or dispense a drop or a bubble. Inertia of the drop may act to oppose the surface tension when the needles are rapidly retracted. This is satisfactory for relatively large drops and where there is low surface tension relative to the drop's inertia, or where some initial velocity upon deployment of the drop, droplet or bubble is acceptable. The use of a pair of matched needles serves to balance out the surface tension between the two needles so that the drop or bubble separates with no or little initial velocity. Usually, there is some slight miss-match in the wetting or the surface tension characteristics of each needle, as well as slight asymmetries in needles geometry and position. Therefore, when the needles are retracted, the drop tends to stick to one needle more than to the other so that asymmetrical forces are applied upon separation. Furthermore, the motion of the needles must be in perfect symmetry in terms of start time and velocity of retraction. Otherwise the drop will remain in contact with one needle longer than with the other resulting in an undesired initial velocity. Also, the needle retraction must be carried out at high speed in order to maximize the effects of inertia and minimize the effects of any asymmetry in retraction. As the size of the drop decreases to droplet size the mismatch in symmetry becomes even more noticeable, in turn resulting in a higher imparted velocity to the droplet.
All of these variables mean that the deployment apparatus must be designed and precisely tuned for each application. Furthermore, the mechanisms for deployment are intolerant to changes in liquid properties or mechanical flaws. The mechanisms are delicate and easily damaged. For these reasons, the techniques lack flexibility thereby limiting their use as a general purpose tool. Furthermore, in space, a common objective is to deploy a droplet or bubble with zero or nearly zero initial velocity. Failure to deploy the droplet or bubble in this manner often results in an experiment which is deemed a failure.
Earth applications for deployment of drops, droplets and bubbles includes the dispensing of liquids such as adhesives and inks or liquid-solid mixtures such as protective coatings, solder pastes, and also molten metal solders. Many operations such as the manufacture of integrated electrical circuits and circuit boards and the dispensing of drops for medical analysis or treatment or for dispensing of biological specimens for diagnosis or research likewise require precise metering and placement of small quantities of fluids.
There are several methods commonly used for dispensing or depositing of fluids including drop dispensing, jet dispensing, drop-on-demand, and spray deposition.
Drop dispensing uses a simple nozzle or needle to form a drop of liquid displaced from a reservoir using a syringe or other pumping or pressurizing method. For drops that are relatively large in proportion to the needle size, gravity opposed by surface tension determines the ultimate size of the drop. When the gravity force exceeds the surface tension, the drop separates from the nozzle. The properties of surface tension and gravity, as well as the needle characteristics are generally fixed and are not easily changed. Therefore, the ability to change the drop size or to control its deployment on command is not practical. For drops which are relatively small (droplets) compared with the size of the needle, the dispensing systems must make physical contact between the droplet and the surface on which the droplet is to be applied. The wetting and surface tension characteristics of the target surface must exceed the surface tension force between the droplet and the needle to assure complete deployment. The need to place each droplet on the target by physically moving the needle or the target up and down, combined with the need to move the needle or target laterally to the next position, slows down the process, adds mechanical complexity, and requires additional controls.
Jet dispensing causes a high speed jet of liquid to issue from a nozzle. The Rayleigh instability causes the liquid to break up into droplets. External vibration is used to create capillary waves in the surface of the jet. These waves are driven by surface tension and grow in amplitude until individual droplets form. This jet dispensing forms the basic approach for many types of ink jet printing. With its high velocities, jet dispensing does not work well for high viscosity fluids or for visco-elastic fluids. Furthermore, the dispensing of liquid drops or bubbles is impossible at low or zero velocities. Also, the process causes a large number of droplets to emanate from the nozzle, on the order of many thousands of droplets per second. Only a small portion of the droplets collect on the target surface, the rest being recycled for reuse. Thus, the efficiency of the system, based upon the percent of deployed droplets striking the target, tends to be very low.
The drop on demand approach is capable of ejecting individual droplets, using a pressure pulse within a liquid cavity to cause each individual drop to be discharged from an orifice. The velocity of the liquid jet in this type of system is high, because of the necessity of overcoming surface tension at the orifice. This system is satisfactorily used only on low viscosity liquids, with high viscosity or visco-elastic materials being unusable.
Spray deposition is similar to jet dispensing except that the flow is distributed over a wide surface area, where the liquid is allowed to break into droplets. The process is usually accompanied by gas jets to accelerate the break up of the flow into droplets. Spray deposition is the basis for conventional spray painting. The system does not work well with highly vicious liquids or visco-elastic liquids. Often, it is necessary to add thinning agents or solvents to reduce the viscosity of the fluid. This results in high levels of vapors, creating health, environmental, and flammability hazards. Furthermore, the wide distribution of droplet sizes makes the discreet dispensing of precisely sized single droplets in a repeatable manner, impossible.
All of these prior art processes suffer similar drawbacks in that they do not permit the dispensing of droplets at a very low velocity. Furthermore, they do not permit the exercise of control over the size, frequency or characteristics of the individual droplets.